1. Field of the Invention
The present invention relates generally to wireless communications and, in particular, to a Random Access CHannel (RACH) information transmission method and apparatus in a wireless communication system.
2. Description of the Related Art
Long Term Evolution (LTE) is a successor to the 3rd Generation (3G) network technologies of the Universal Mobile Telecommunication Service (UMTS) standard, and supports high-speed packet services based on Orthogonal Frequency Division Multiplexing (OFDM).
Referring initially to FIG. 1, a diagram illustrates an LTE system architecture.
LTE mobile communication is characterized by Evolved Radio Access Networks (E-RANs) 110 and 112 having only two types of infrastructure nodes: Evolved Node B's (ENBs) 120, 122, 124, 126, and 128 and anchor nodes 130 and 132. A User Equipment (UE) 101 accesses an Internet Protocol (IP) network 114 via E-RANs 110 and 112. The ENBs 120, 122, 124, 126, and 128 are responsible for wireless channel establishment for the UE 101 and management of the cells and wireless resources. For example, the ENBs 120, 122, 124, 126, and 128 broadcast system information, allocate a radio resource for transmission of data and control information to the UE 101, and determine a handover of the UE 101 based on channel management information collected from a current cell and its neighbor cells. The ENBs 120, 122, 124, 126, and 128 are provided with a control protocol such as a Radio Resource Control (RRC) related to the radio resource management.
FIG. 2 is a diagram illustrating RACH signaling between a UE and an ENB in a conventional LTE system.
In FIG. 2, a UE 201 attempts random access to an ENB 203. The ENB 203 broadcasts system information including information on parameters specific to random access in step 211. The system information includes information on ranges of IDs of Random Access Preamble group A and Random Access Preamble group B, a transmission message size threshold of the UE 201 (THRES), and a channel condition offset (OFFSET). These parameters are specified in the 3GPP standard TS36.331v850. The information on the range of IDs of preambles in Random Access Preamble group A is informed by sizeOFRA-PreamblesGroupA. The information on the range of IDs of preambles in Random Access Preamble group B is obtained by numberOF-Preambles (total number of preambles for contention-based Random Access)-sizeOFRA-PreamblesGroupA. MessageSizeGroupA is an offset indicating a channel condition. When a pathloss is less than PCMAX-preambleInitialReceivedTargetPower-deltaPreambleMsg3-messagePowerOffsetGroupB (i.e., the channel condition is good) and the transmission size is greater than messageSizeGroupA, the UE selects a preamble in the Preamble Group A. In other cases, the UE selects a preamble in Preamble Group B. The parameters PCMAX, preambleInitialReceivedTargetPower, and deltaPreambleMsg3 are specified in 3GPP standard TS36.331v850.
Once the preamble group and a preamble are selected, the UE 201 sends the selected random access preamble to the ENB 203 on a RACH in step 221. If the random access preamble is received successfully, the ENB 203 sends a random access response message containing information on the preamble ID and Timing Advance (TA) for adjusting the uplink timing in step 231. Upon receipt of the random access response message, the UE sends a scheduled transmission message containing information on the uplink resource allocation for message transmission and Temporary-Radio Network Temporary Identifier (T-RNTI) to the ENB 203 in step 241.
If more than one UE used the same random access preamble simultaneously at step 221, they are likely to collide with each other. In order to make clear which UE has succeeded in transmitting the preamble, the ENB 203 sends a contention resolution message containing information on the Serving-Temporary Mobile Subscriber Identity (S-TMSI) received at step 241 or a Random number to the UE in step 251. Each of UEs that used the same preamble receives the contention resolution message and checks whether the S-TMSI or Random number matches that which it transmitted at step 241. If the ID information matches, the UE continues the random access procedure and, otherwise, reinitiates the random access procedure.
In the LTE system, the size of the message transmitted by the UE 201 at step 241 of FIG. 2 is limited. Thus, the UE transitioning from an idle mode to a connected mode can transmit only an RRC Connection REQ message at step 241 and a Non Access Stratum (NAS) Service Request (REQ) message after step 251. Since the RRC Connection REQ message and the NAS Service REQ message are transmitted in stepwise manner, this causes the procedural delay. The RRC Connection REQ message and the NAS Service Request message are defined in the 3GPP standards TS36.331v850 and TS24.301v810, respectively.